Blood Pressure (BP) measurement is a physiological indicator that has found wide spread use in both clinical and non-clinical settings. Usage scenario of BP poses unique functional requirements and challenges. Conventional cuff less method uses two major principles such as tonometry and pulse transit time to measure blood pressure.
In clinical practice, an applanation tonometry is often used which requires an operator to suppress arterial pulsations by externally applied pressure to capture an arterial pressure accurately. The measurement is influenced by an external applied pressure and requires skill to perform correctly. While tonometry is commonly practiced in ophthalmology, its use in measuring arterial blood pressure has remained confined mostly to tertiary clinical care settings and research applications, owing to relatively high cost of devices and skill required to perform accurate measurements.
Devices such as SphygmoCor (AtCor Medicals) utilize tonometry on radial, carotid and femoral arteries along with upper arm cuff measurements for evaluation of central aortic pressure and arterial stiffness by pulse contour analysis. A modification of tonometry principle, integrating an automated mechanism to perform applanation is referred to as vascular unloading and is used by a Finapres device. However, the measurement is performed using photoplethymography sensors on a fingertip and model based techniques are used to reconstruct brachial and aortic pressures, and calibration with an upper arm cuff is required for accurate measurements.
Pulse transit time (PTT) based techniques are the most widely researched in the area of cuff-less BP measurement. The method is based on the relation between BP and PTT, measured by evaluating the time taken by the blood pulse to propagate across a section of the arterial tree. While the basic instrumentation required for this is relatively simple, utilizing dual Photo Plethysmo Graph (PPG) waveforms or a single PPG waveform and ECG signal, the estimation of blood pressure is often performed by model based computation that requires calibration. Most of the existing method of blood pressure measurement uses Moens-Korteweg equation describing the relationship between Pulse Wave Velocity (PWV) and pulse pressure. The PTT based techniques are most commonly used and hence the requirement of population and patient specific calibration limits measurement accuracy, reliability and widespread use.
Although PTT based techniques have the advantage of reduced instrumentation complexity, affordable cost and amenability for a wearable device design, the need of patient specific and population specific calibration to evaluate the constants used in estimation of blood pressure limits measurement accuracy and utility in practice. The requirement of an Electrocardiography (ECG) measurement to accurately measure the PTT is limitation in most systems that need to be addressed to enable wide spread use of the PTT based methods. It may also be remembered that the fundamental relation between the PTT and pressure as described by the Moens-Korteweg or the Bramwell-Hill equation is valid under assumptions of elastic artery walls excited by pulsating pressure with no wave reflections. This is not the case when measurements of PTT are performed at two different points along the arterial tree that has an arterial branching in between. Further, both the viscoelastic nature of vessels and effect of wave reflections tend to affect measurements performed on a peripheral vascular tree (such as radial artery or fingertip) which is the case with most reported systems.
The above information is presented as background information only to help the reader to understand the present invention. Applicants have made no determination and make no assertion as to whether any of the above might be applicable as Prior Art with regard to the present application.